This 8-week clinical trial investigated the effects of Zn supplementation on the Zn status, liver aminotransferases, anthropometric variables, glycemic indices, lipid profile, and some inflammation and oxidative stress parameters in patients with NAFLD. According to the between-group comparison, consuming 30 mg/d Zn had a significant effect on the Zn status, anthropometric parameters, AST, TC, and LDL-C, but no significant effect on ALT, FBS, insulin, HOMA-IR, HDL-C, TG, hs-CRP, MDA, and TAC.

The effects of Zn on the serum zn

The liver is the storage organ for Zn, and the relationship between Zn and the liver is a cooperative pathway. Zn deficiency is common in NAFLD patients and also causes liver dysfunction [12]. In this study, the mean serum Zn in the intervention group increased significantly. This value decreased in the placebo group, but this decrease was not statistically significant. In general, the mean changes in the serum Zn in the intervention group compared to the placebo were statistically significant. Consistent with our study, receiving Zn supplement caused a significant increase in the serum Zn in obese patients with NAFLD in the previous study [12]. Also, based on the results of previous studies on various diseases such as DM [11], metabolic syndrome [13], obesity [10], and polycystic ovary syndrome [14], Zn supplementation increased the serum Zn in the intervention group, which is consistent with the present study. Factors and mechanisms that reduce Zn in patients with NAFLD include inflammation, oxidative stress, obesity, hypertension, DM, IR, impaired gastrointestinal absorption of Zn, and decreased Zn intake [15].

The effects of Zn on the liver aminotransferases

Liver enzymes such as ALT and AST were evaluated as markers of NAFLD evaluation in this study. According to the results, Zn supplementation significantly reduced AST, but it had no effect on ALT in NAFLD patients. Furthermore, the results of the study of Fathi et al. were in line with those of our study [12]. The results of a study by Murakami et al. showed that the decrease in ALT due to Zn intake in patients with hepatitis C might be due to the antioxidant role of this element [16]. However, in the study of Idowu et al., the mean of ALT and AST enzymes in the group that received Zn supplementation was higher than the control group [17]. Although Zn is expected to have a beneficial effect on liver enzymes, some conflicting results might be due to long-term and high-dose use as some studies have shown that high doses can be harmful to the hepatocyte membrane by increasing ALT and AST [18].

The effects of Zn on the anthropometric parameters

Within-group and between-group comparisons indicated a significant decrease in weight, waist circumference, and BMI parameters in the present study. Consistent with our study, the results of previous studies indicated that Zn supplementation improved anthropometric parameters [19,20,21]. However, the effect of the Zn supplementation on waist circumference was not significant in some studies [13, 14]. Differences in the intervention dose, age group, studied disease, and baseline of mean serum Zn could be the reasons for some inconsistencies with the results of the present study. Zn can improve anthropometric indices by improving the IR index, having insulin-like effects, and enhancing lipid-related metabolic pathways. Zn also increases leptin, which can affect and reduce food intake by producing or inhibiting the production of some appetite mediators. In addition to affecting food intake, leptin increases energy expenditure by stimulating the sympathetic nerves. Besides, leptin rises the expression of the UCP-1 protein-producing gene by stimulating brown adipose tissue and ultimately increases the exothermic effect. Leptin can also increase insulin sensitivity and reduce TG accumulation in the skeletal muscle and liver tissue, regardless of its effect on diet and weight [22,23,24].

The effects of Zn on the lipid profile

Dyslipidemia is a common disorder in NAFLD patients that increases the risk of CVD. Based on the results of the present study, the improvement of lipid profile was significant in the intervention group, but not in the control group. Between-group changes were also significant for TC and LDL-C, but not for HDL-C and TG. In the same line with the present study, Zn supplementation in some studies led to significant effects on the lipid profile [8, 25]. Conversely, for some components of the lipid profile, the results of some studies were not consistent with those of the present study [11, 26]. Different doses and duration of intervention may be the main reasons for inconsistent results in our study. Zn has insulin-like effects in the body that can enhance lipogenesis and glucose transport into the cell. As a result, this function can improve the lipid profile and proper function of cells [25, 27, 28]. Another possible mechanism is related to the effect of Zn supplementation on leptin production. Leptin directly improves the lipid profile by increasing insulin sensitivity and reducing TG accretion in the skeletal muscle and liver tissue. Leptin also indirectly improves the lipid profile by reducing food intake and weight [22,23,24].

The effects of Zn on the glycemic index

Although Zn supplementation significantly reduced glycemic index (FBS, insulin, and HOMA-IR) in the intervention group, no significant difference was observed in the mean changes of these parameters between the two study groups. Consistent with the present study, the levels of IR [7, 13] and insulin [7] in the intervention group were significantly reduced in previous studies. Unlike the present study, the decrease in insulin, FBS, and IR in the intervention group of the previous study was not significant [29], and insulin levels and IR increased in the placebo group [7]. Zn is an essential element for the processing, storage, and secretion of insulin in the pancreatic beta cells, which are significant reservoirs of Zn. On the other hand, Zn has an insulin-like effect on the body’s cells and can have similar effects even without the presence and function of insulin in the body [28, 30]. The effect of Zn on insulin signaling is related to the stimulation of several compounds including phosphoinositide-3 kinase, phosphorylation of tyrosine from insulin receptor β subunit, and phosphorylation of tyrosine in insulin receptor substrate-1 (IRS-1), and phosphorylation of serine-473 in AKT [28, 31,32,33]. Zn also indirectly affects the insulin-like growth factor (IGF) via its insulin-like function. Other insulin-like mechanisms of Zn include the inactivation of the glycogen synthase kinase-3β (GSK-3β) enzyme. This enzyme is a protein kinase related to IR, and Zn inactivates GSK-3β by the PI3 / AKT signaling pathway. Moreover, Zn has beneficial effects on IR due to leptin. Various studies have shown that leptin depletion can be considered a factor in increasing IR as described in the previous part [22, 23].

The effects of Zn on the hs-CRP

Inflammation causes damage to the hepatocytes and the progression of liver disease [34]. Based on the results of the present study, consuming the Zn supplement had no significant effect on the hs-CRP level. Although the results of some studies were in line with those of our study [10, 14], some studies showed inconsistent results and Zn intake reduced the level of hs-CRP [13, 35, 36]. Gut-liver axis can play an effective role in liver disease and increase inflammatory factors [37]. According to studies, Zn supplementation can have positive effects on the gut-liver axis by reducing endotoxemia, reducing oxidative stress and the production of inflammatory cytokines, stabilizing the intestinal defense barrier, and exerting positive effects on hepatocyte apoptosis [38, 39]. Foster and Samman in their study found that Zn supplementation in higher doses equal to 45 mg/d can reduce pro-inflammatory factors. However, at low doses, the effects were different, so even at doses less than 10 mg/d, they had the opposite effect and increased the inflammatory mediators. Consequently, they reported a dose-dependent response of inflammatory factors to the Zn supplementation [40]. As a result, it can be said that one of the possible reasons for the insignificancy of Zn supplementation on inflammatory factors and also discrepancies in the results of different studies in comparison with our research could be the selected dose of 30 mg/d in our study and different doses in other studies.

The effects of Zn on the oxidative stress

MDA as a marker of fat oxidation and TAC are components of oxidative stress that play an important role in NAFLD. Although we observed a decrease in the MDA level and an increase in TAC in this study, these changes were not statistically significant. While some previous studies were in line with the present study and Zn supplementation did not have a significant effect on MDA [36, 41], in some studies, this factor was significantly reduced [13, 14]. Regarding the TAC, the results of a previous study were similar to those of our study [14], while in some other studies a significant increase in TAC was observed by means of Zn intake [10, 36]. Zn increases the antioxidant activation of proteins, molecules, and enzymes such as glutathione, catalase, and superoxide dismutase and reduces the activity of oxidant-promoting enzymes such as nitric acid synthetase [42, 43]. The difference in the duration of the intervention, the dose of supplementation with Zn as well as the serum Zn level in the studies are likely to be the possible reasons for not relevant effect of Zn supplementation on oxidative stress factors.

Strengths and limitations of the study

Measuring serum Zn and controlling the confounding effect of food intake by prescribing both groups a low-calorie diet were the strengths of this study. The short duration of the study and small sample size were the limitations of this study. Furthermore, the results of this study cannot be generalized to the entire society and these results should be assessed by future studies.